Add redisConnectFd() and redisFreeKeepFd()

These allows for easier integration of hiredis with external
code that wants to manage its fds, say for instance in a pool.
Add test for inherited/kept fd.

This patch provides coverage for redisConnectFd() and
redisFreeKeepFd(). Since we're really only interested
in the connection side of the code, we only call
test_blocking_connection() to make sure the connection
works.

Passing --skip-inherit-fd will disable the test.
Add -e to the echo to expand newlines.